Merge resolved this by shipping the SDK on pydantic 2.0 (collaborator dsinghvi confirmed the migration was done). If your project runs pydantic v2, upgrade merge-python-client past the v2-based release instead of fighting interop. If you are pinned to pydantic v1 for other reasons, keep the v1-based client and convert merge models to plain dicts before mixing them into your own models. Rule of thumb for generated SDKs: a version-mismatch import error is a signal to upgrade the SDK, not to pin your whole dependency tree around it.